Eddy-current brake device

ABSTRACT

An eddy-current brake device includes at least one heat exchanger for dissipating the thermal energy that is due to the eddy currents during braking. The heat exchanger is designed to define a path for the cooling liquid, which path is predetermined in order to minimize the pressure drops and maximize the rate of flow of cooling liquid circulating through the exchanger, with a view to reducing the variations in working temperature of the heat exchanger. The device may include two symmetric heat exchangers, of which the inlets, the circuits and the outlets for water are arranged symmetrically, so as to compensate for the forces due to the currents of cooling liquid and so as thus to minimize the corresponding residual torque.

BACKGROUND OF THE INVENTION

The invention relates to an eddy-current brake device of the typecomprising at least one heat exchanger for dissipating the thermalenergy that is due to the eddy currents during braking.

To measure engine performance, particularly the performance of internalcombustion engines, use is made of braking devices connected to theoutput shaft of the combustion engine.

In eddy-current brake devices, a toothed rotor is driven by the outputshaft of the combustion engine to be tested. This toothed rotor, whichis rotated, has teeth located at the periphery which cut the force linesof the magnetic field created by a coil. The cutting of the magneticfield by the teeth of the rotor and the gap between the lateral faces ofthe teeth and the adjacent metal parts give rise to eddy currents in theconducting material located on each side of the toothed rotor. Theseeddy currents generate a substantial amount of thermal energy in thismaterial, which thus has to be cooled in order to remove the heat energygenerated by the eddy currents.

To this end, in a known way, heat exchangers are placed on each side ofthe toothed rotor: these heat exchangers are cooled by a significantflow of cooling liquid.

Bearing in mind the significant braking effect due to the eddy currents,an eddy-current brake can be considered as being a device which convertsmechanical braking energy into heat energy that has to be dissipated inthese heat exchangers. These heat exchangers are subjected, when thecombustion engine undergoing test changes speed and especially when itchanges load, to very substantial temperature variations: hence, thematerial of the exchangers experiences temperature jumps from 20° C. to400° C. as the speed of and load on the combustion engine increases.These temperature jumps give rise to thermal fatigue of the material ofthe exchangers, which leads to the appearance of cracks and to swiftdeterioration of the exchangers.

Numerous heat exchanger systems or principles for eddy-current brakeshave been proposed, without yet being able to achieve sufficientendurance or reliability to allow for long-term use.

The mechanical, thermal and hydraulic stresses have led to a preferredtype of exchanger in which the path of the cooling liquid involvessignificant pressure drops, which means that it is not possible toincrease the rate of flow of cooling liquid through these exchangers.

In general, cooling water is used by way of preferred cooling liquid.This cooling water enters via an inlet located at the periphery of theexchanger and re-emerges via an outlet located toward the center of theexchanger.

SUMMARY OF THE INVENTION

A first object of the invention is to propose a new heat exchangerarrangement for eddy-current brakes, so that the rate of flow of coolingwater can be increased and the temperature level experienced by thematerial of the exchanger can thus be decreased.

A second object of the invention is to allow long-term operation of aneddy-current brake device, to improve the availability of these devicesand the length of time for which they may be used.

A third object of the invention is to reduce the pressure dropsexperienced by the cooling liquid as it circulates through theexchanger.

A fourth object of the invention is to propose an eddy-current brakedevice configuration in which the precision and quality with whichengine torque is measured are increased, by virtue of a decrease in thetorque induced by the cooling water circuits.

A fifth object of the invention is to propose a heat exchangerconfiguration that can replace heat exchangers for eddy-current brakesof the known type.

A subject of the invention is an eddy-current brake device, of the typecomprising at least one heat exchanger for dissipating the thermalenergy that is due to the eddy currents during braking, characterized inthat the heat exchanger is designed to define a path for the coolingliquid, which path is predetermined in order to minimize the pressuredrops and maximize the rate of flow of cooling liquid circulatingthrough the exchanger, with a view to reducing the variations in workingtemperature of the heat exchanger.

The device according to the invention comprises two symmetric heatexchangers, of which the inlets, the circuits and the outlets for waterare arranged symmetrically, so as to compensate for the forces due tothe currents of cooling liquid and so as thus to minimize thecorresponding residual torque.

According to other features of the invention:

the path of cooling liquid is predetermined so that the circulation ofthe cooling liquid is practically constantly in one and the samedirection of winding,

the path of the cooling liquid may comprise several sections, forexample of rectangular profile or of rectangular and semi-toroidalprofile, connected together by connecting ducts; or, alternatively, thepath of the cooling liquid may be shaped approximately in the form of aspiral,

the path of the cooling liquid may, in another embodiment, be producedat least partially by winding a tube of approximately constant crosssection,

the circulation of liquid in each of the two exchangers is preferablyapproximately about the axis of rotation of said rotor, the directionsof circulation in said two exchangers being the opposite of one another.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood by virtue of the descriptionwhich will follow, given by way of nonlimiting example with reference tothe appended drawings, in which:

FIG. 1 schematically depicts a perspective view of an eddy-current brakedevice,

FIG. 2 schematically depicts a view in part section on a vertical planepassing through the axis of rotation of the toothed rotor of aneddy-current brake device,

FIG. 3 schematically depicts an elevation in the direction of arrows IIIof FIGS. 1 and 2 of an eddy-current brake device,

FIG. 4 schematically depicts a partial view in elevation and a view insection on IV—IV of a heat exchanger of a known type for an eddy-currentbrake,

FIG. 5 schematically depicts a part view in elevation with section onV—V of a heat exchanger according to the invention,

FIG. 6 schematically depicts a comparative diagram of temperatureprofile as a function of engine speed of a combustion engine to betested,

FIG. 7 schematically depicts a part view in elevation and a part view insection on VII—VII of another heat exchanger according to the invention,

FIG. 8 schematically depicts a view in section of an eddy-current brakedevice according to the invention, with two heat exchangers,

FIG. 9 schematically depicts a perspective view of an eddy-current brakedevice according to the invention, with two symmetric heat exchangers,

FIG. 10 depicts a diagram showing the influence of supply pressure ontorque measurement,

FIG. 11 depicts a plan view of the layout of the heat exchangersaccording to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIGS. 1 to 3, elements which are identical orfunctionally equivalent are labeled with identical reference figures.

In FIG. 1, an eddy-current brake device is mounted on two assemblies 1and 2 forming a rotation bearing. Each bearing 1 or 2 comprises amounting plate 1 a or 2 a for attachment to a bed and a support 1 b or 2b. The supports 1 b and 2 b consist of oscillating bearings supportingthe body 3 of the device.

The body 3 comprises an extension 4 constituting an anti-rotation armwhich is connected to an element 5 secured to a stationary bed by a loadsensor 6 that allows the braking torque generated by the eddy-currentdevice to be determined.

A shaft 7 intended to be coupled to a combustion engine is mounted torotate freely inside the body 3 of the device, from which it protrudesat a rear end and at a front end 8 consisting of a flange for couplingto a combustion engine, not depicted.

A peripheral cooling-water inlet 9 provides the supply for a heatexchanger internal to the body, while an outlet 10 near the center ofthe body 3 allows this cooling water out. As a preference, the devicehas two water inlets 9 and two water outlets 10: the cooling-waterinlets 9 are connected together and coupled to a common supply 11, whilethe water outlets 10 are connected together and coupled to a commondischarge 12.

Because the cooling-water inlets 9 and the cooling-water outlets 10 arelocated on different radii and are inclined with respect to these inletand outlet radii at a large angle of about 45 degrees of angle, residualtorque occurs as a result of the circulation of the cooling water.

The value of this torque, measured by the load sensor 6 in the absenceof any engine to be measured, varies according to variations in thepressure, the flow rate of the water, and the internal pressure dropswithin the exchangers. This residual torque in fact leads to ameasurement error in subsequent measurements of torque on the enginesthat are to be measured.

In FIG. 2, the shaft 7 for coupling to the combustion engine, notdepicted, carries a toothed wheel 13 that allows the rotational speed tobe measured using a tachometric sensor 14 fixed to the support 1 b. Theshaft 7 intended to be coupled to the combustion engine, not depicted,is mounted to rotate freely on rolling bearings 15 a, 15 b, 15 c, andalso carries a rotor 16 rotated by the shaft and carrying teeth 16 athat cut the magnetic flux generated by a magnetic coil 17 located atthe periphery. The body 3 of the brake device oscillates on the bearings1 and 2 and comprises two central parts 18 and 19 made of light alloyand two plates 20 and 21 carrying the coil 17, spaced apart by acylindrical part 22. The plates 20 and 21 act as supports for twoexchangers 23 and 24 located one on each side of the rotor 16 rotated bythe shaft 7 while the combustion engine is being tested.

When the forces lines of the magnetic field created by the magnetic coil17 are cut by the toothed rotor 16 rotated by a combustion engine thatis to be tested, eddy currents develop mainly in the material of theexchangers 23 and 24, the temperature of which rises instantly.

The righting torque exerted by reaction on the body 3 of the device isdetermined by measuring the load applied to the load sensor 6 located ata known distance from the extension 4 which forms a lever arm of knownradius. This thus gives the engine torque of the combustion enginedriving the rotation of the shaft 7, while the speed of this combustionengine is measured by the tachometric sensor 14 placed facing thetoothed wheel 13. Determining the engine torque and the engine speedallows the power of the combustion engine to be determined instantlyduring the testing.

Bearing in mind the heating of the material of the exchangers 23 and 24,these exchangers have a great thickness and rectangular ducts ofpredetermined cross section so as to limit the mechanical deformationsand maintain good dimensional stability as they undergo successiveheating and cooling.

Although the invention is described with reference to a devicecomprising a radially external cyclindrical spacer 22, it is not in anyway restricted to this particular embodiment and also covers any othertype of eddy-current brake device, for example devices comprising twocoils placed on the outside of the body and allowing air trapped at thetop of the teeth of the rotors to be removed.

In FIGS. 2 and 3 it can be seen that the protruding end of the shaft 7is protected by a cap 25 attached by appropriate screws.

With reference to FIG. 4, a part 23 a of a heat exchanger of known typeis depicted in solid line, while its cover plate 23 b, which may also beproduced directly in the plates 20 and 21, is depicted in broken line.The part 23 a is made of very thick plate, preferably by machining, soas to have holes 30 for positioning and attachment to the plate 23 bforming a cover and so as to have rectangular water-circulation ducts31.

The water inlet and the water outlet are both located on the samediameter: the water inlet 9 is located on the diametrical axis of theoutermost circular groove, while the water outlet 10 is located on theinnermost circular groove. Each water-passage groove 31 is connected tothe next water-passage groove by a passage 32 made in a partition 33, soas to define semicircular paths for the cooling water.

The water thus enters by the inlet 9, each half of the flow spreadingout across each part of the first circular groove 31 to make half aturn, the two halves meeting at the first passage 32 and splitting againhalf into each of the next half-turns in the next circular groove 31 andso on, as far as the water outlet 10. This embodiment makes it possibleto obtain an exchanger construction which has good rigidity and goodstability to repeated deformation brought about by thermal expansion.The water may also be circulated in the opposite direction, that is tosay entering via the passage 10 located on the most central groove andleaving via the groove 9 located furthest toward the outside. However,that method of circulation has the drawback of passing approximatelyhalf the totalflow of water through each exchanger half-turn, and alsothe drawback of a sharp pressure drop at the changes of direction of thewater flow at each passage 32. This then results in a limitation in therate at which the cooling water can flow through the exchanger, becauseany attempt at increasing the flow rate very soon leads to turbulentflow which can no longer effectively cool the exchanger when thecombustion engines under test experience a rapid rise in speed or load.

These drawbacks are also encountered if attempts are made at reversingthe direction of flow of water by causing the water to enter via thepassage 10 and removing it via the passage 9.

With reference to FIG. 5, part 40 of the exchanger can replace part 23 aof known type to form, with a cover plate 23 b of known type, anexchanger according to the invention.

The exchanger part 40 according to the invention is constructed todefine a predetermined path for the cooling water, minimizing pressuredrops by comparison with the known exchanger described with reference toFIG. 4 and maximizing the flow of coolant circulating through theexchanger, with a view to reducing the variations in working temperatureof the exchanger.

To this end, the exchanger part 40 according to the invention has anumber of circular ducts 41 of rectangular cross section.

The circular ducts 41 thus have, at least locally, a toroidal shape ofrectangular or rectangular and semi-toroidal cross section.

The invention proposes a cooling water path designed to minimizepressure drops, because the circulation of cooling water is practicallyconstantly in one and the same direction of winding. To this end,provision is made for the toroidal cross sections defined by thecircular ducts 41 to be connected in such a way as to define a constantdirection of circulation in one and the same direction of winding byvirtue of the fact that the cooling water is ducted through passages 42,42 a and 42 b. The passages 42 are passages made in the ribs 43 thatremain by machining in the direction of flow of the cooling water so asto minimize the pressure drops. The passages 42 are machined over theentire height of the ribs 43. The passages 42 a and 42 b are mademid-way up the rib 43 a, and are also machined in the direction of flowof the cooling water. Each passage 42 a or 42 b is defined to allowapproximately half the nominal flow rate of water flowing through agroove 41 to pass.

Fixing orifices 44 identical to the positioning and fixing orifices 30of FIG. 4 are also provided, to allow interchangeability of a part 23 aof known type, with a part 40 of an exchanger according to theinvention.

In order to avoid any counter-flow circulation of cooling water, thetoroidal ducts 41 are advantageously blocked with pegs 45 located nearthe passages 42.

The pegs 45 are preferably pushed into machinings or drillings made forthat purpose, it being emphasized that the blocking achieved by each peg45 placed in a corresponding duct 41 is over practically the entireheight of the duct 41.

Thus, by virtue of the invention, a heat exchanger for an eddy-currentbrake device can be produced very simply by circular machining ofgrooves 41, oblique milling of passages 42, 42 a and 42 b, and by usingcylindrical pins 45 to block the ducts 41 machined in very thick plate.

The cooling water enters, for example, via a water inlet 46 located atthe same place as the water inlet 9 described with reference to FIGS. 1to 4 and leaves, for example, via an outlet 47 located at the same placeas the water outlet 10 described with reference to FIGS. 1 to 4.

It is also possible to envision an embodiment analogous with that ofFIG. 5, by reversing the direction of flow of water, without departingfrom the scope of the present invention.

With reference to FIG. 6, a comparative diagram indicates the differencein water temperature at the inlet and at the outlet of two heatexchangers for an eddy-current brake device.

The upper curve C is a test curve produced using an exchanger of knowntype described with reference to FIG. 4, while the lower curve I isproduced using a heat exchanger according to the invention, describedwith reference to FIG. 5.

A comparison of the two curves demonstrates the fact that the increasein engine speed, at constant load, from 1500 rpm to 6000 rpm gives riseto an increase in temperature difference between the outlet and theinlet which rises from 45° C. at 1500 rpm to 105° C. at 6000 rpm. Thisvariation in temperature difference is testimony to the significantheating in the heat exchanger of known type described with reference toFIG. 4.

By contrast, by virtue of the invention, the temperature differencebetween the outlet and the inlet rises from 30° C. at 1500 rpm to 55° C.at 6000 rpm which appreciably reduces the fatigue due to thermalexpansion of the heat exchanger according to the invention describedwith reference to FIG. 6.

This advantageous arrangement thus allows an eddy-current device to beused for a longer length of time using an exchanger according to theinvention, due to the greater life of the heat exchanger according tothe invention.

The invention extends not only to the case of eddy-current brake devicesincorporating the heat exchanger according to the invention, but also toheat exchangers for eddy-current brake devices considered as spare partsor replacement parts for an exchanger of known type.

With reference to FIG. 7, another embodiment according to the inventionof a heat exchanger for an eddy-current brake device comprises a part 50consisting of a square or rectangular tube 51 bent into the form of aspiral.

The tube 51 preferably has a thickness greater than 2 mm, so that it hasgood rigidity and low deformation in bending.

To avoid any lateral deformation of the spiral, two thick supportingdiscs 52 and 53 may advantageously be provided, one on each side of thespiral-wound tube. This heat exchanger according to the invention hasthe advantage of being simple and economical to manufacture, while atthe same time allowing all of the water flow to pass and minimizing thepressure drops as it does so. To further reduce the pressure drops it ispossible to envision a water inlet 54 approximately tangential to thespiral winding and a cooling-water outlet 55 approximately tangential tothe outside diameter of the spiral winding.

It is also possible, without departing from the scope of the presentinvention, to reverse the direction of circulation.

Finally, in order to produce a spiral-shaped path for the water, it ispossible to machine a spiral groove directly in thick plate using anumerically controlled machine.

However, this machine is more difficult because the machining depth isvery much greater than the width of the machined groove.

The spiral-shaped groove that remains after machining needs to have aprofile which has a very strong base to avoid any cracking as theexchanger goes through thermal cycling.

With reference to FIG. 8, an eddy-current brake device according to theinvention comprises two heat exchangers according to the invention withthe water circulating in a spiral.

The two exchangers are preferably identical, so as to reduce their costsof manufacture and are arranged one on each side of the rotor 16,rotated through 180° one with respect to the other about an axis roughlyperpendicular to the axis 60 of the device. As a preference, they areapproximately symmetric with each other with respect to a vertical axispassing through the center A of the rotor 16. The directions ofcirculation of the cooling water in the two exchangers are thus opposed,which allows the forces and the moments of the forces exerted on thebrake device by the circulation of water through the two exchangers topractically compensate one for the other, give or take the differencesin pressure drop between the two exchangers.

The water inlet 56 of the first exchanger and the water inlet 57 of thesecond exchanger are symmetric with one another with respect to avertical plane approximately perpendicular to the axis of rotation 60 ofthe rotor 16. Likewise, the water outlet 58 of the first exchanger andthe water outlet 59 of the second exchanger are symmetric with respectto this plane.

Because of the symmetry of the forces due to the water inlets andoutlets of the exchangers and because of the symmetry of thecirculations of water through the two exchangers, no appreciableresidual torque is detected by the load sensor 62 mounted between theapproximately horizontal anti-rotation arm 61 and an element 63 securedto the bed, not depicted.

This advantageous arrangement of the water inlets 56, 57 and the wateroutlets 58, 59 of the exchangers according to the invention with coolingcircuits in the form of symmetric spirals, entails difficult machiningof spiral-shaped grooves using a numerically controlled machine.

With reference to FIG. 9, another embodiment of the device according tothe invention comprises two heat exchangers symmetric with one anotherwith respect to a vertical axis passing approximately through the centerA of the rotor 16.

This device comprises a water inlet box 71 supplying the water inlet 72of the first exchanger and the water inlet 73 of the second exchanger,and a water outlet box 74 receiving the water leaving the water outlet75 of the first exchanger and the water outlet 76 of the secondexchanger. The water inlets 72 and 73 are symmetric with one anotherwith respect to a vertical axis passing approximately through the centerA of the rotor 16. Likewise, the water outlets 74 and 75 are symmetricwith respect to the axis passing approximately through the center A ofthe rotor 16 and perpendicular to the horizontal plane of measurement ofthe anti-rotation arm, not depicted.

The boxes 71 and 72 are mounted joined together and are fixed to amounting plate 77 for mounting on a support, not depicted.

The symmetrical arrangements of the cooling-water circuits has theresult of compensating for the torques that vary with variations inpressure, water flow rate, and corresponding pressure drops.

With reference to FIG. 10, a comparative diagram indicates the residualtorque of the device as measured by the load sensor, in the absence ofany engine to be measured, as a function of the cooling water supplypressure.

When the supply pressure varies from 1 to 3 bar, the curve C_(R)relating to a device of known type shows a variation in residual torquefrom +1.18 Nm to −1.35 Nm, namely an amount of variation of 2.53 Nmmeasured by the load sensor.

By contrast, when the supply pressure varies from 1 to 3 bar, the curveI_(R) relating to the device according to the invention of FIG. 9indicates a variation in residual torque from −0.1 Nm to −0.12 Nm,namely an amount of variation of 0.02 Nm measured by the load sensor.

With reference to FIG. 11, an exchange according to the inventionintended for a device according to the invention of the kind depicted inFIG. 9 comprises a part 80 designed to define a predetermined path forthe cooling water, in the direction of winding corresponding to thecirculation arrows, in the ducts 81 a to 81 e.

The ducts 81 a to 81 e at least locally have an annular shape ofrectangular section or of rectangular and semi-toroidal cross section.

The circular ducts 81 a to 81 e are connected by passages 82 a to 82 dmachined in the ribs that remain between two consecutive ducts.

Thus, the cooling water entering via a radially inner water inlet 83,supplied, for example, by inlet 72 of FIG. 9, passes through the ducts81 a to 81 e in the same direction of winding, passing through thepassages 82 a to 82 d and re-emerges via a radially outer water outlet84 to pour, for example into the outlet 75 of FIG. 9. The water may alsobe circulated in the opposite direction, that is to say may enter viathe radially outer passage 84 and leave via the radially inner passage83 without departing from the scope of the present invention.

To minimize the distance d separating the vertical projections of thewater inlet 83 and of the water outlet 84, the locations 85 a to 85 e ofthe plugging pegs are arranged in a radial alignment, for example alongthe vertical.

The fact of minimizing the distance d allows the hydraulic mode ofoperation of the device of FIG. 9 comprising two symmetric heatexchangers according to FIG. 11 to be brought as close as possible tothe hydraulic mode of operation of the exchanger of FIG. 8 so as to makethe variations in residual torque I_(R) of the device in the absence ofan engine to be measured negligible, as was described with reference toFIG. 10.

However, obtaining a minimized distance d with reference to FIG. 11,entails making passages 82 a to 82 d of a width smaller than the widthof the ducts 81 a to 81 e; although this modification with respect toFIG. 5 of the machined widths of the passages locally alters thehydraulic conditions by increasing the rates at which water passes andincreases the total pressure drop in the exchanger, this modificationdoes not constitute an appreciable drawback with regard to the obtainedreduction in residual torque and to the increase in the precision andquality with which the torque of engines to be measured can be measured.What happens is that the measurement precision is close to the observedvariation in residual torque; this precision is therefore, by virtue ofthe invention, about one hundred times better than the precision of theprior art.

The invention described with reference to a number of particularembodiments is not in any way restricted thereto and on the contrarycovers any modification in form and any variant form of embodiment thatfalls within the scope and the spirit of the invention.

What is claimed is:
 1. Eddy-current brake device, comprising two heatexchangers for dissipating the thermal energy that is due to eddycurrents during braking, wherein each heat exchanger is designed todefine a path for circulating a cooling liquid approximately about anaxis of rotation of a rotor of said device, said path is predeterminedin order to minimize the pressure drops and maximize the rate of flow ofcooling liquid circulating through the exchanger for reducing thevariations in working temperature of the heat exchanger, and wherein thedirections of circulation in said two heat exchangers are opposite ofone another so as to compensate for the forces due to currents of thecooling liquid and to minimize a corresponding residual torque. 2.Device according to claim 1, characterized in that the path (41, 51, 81a-81 e) of cooling liquid through an exchanger is predetermined so thatthe circulation of the cooling liquid is practically constantly in oneand the same direction of winding.
 3. Device according to claim 2,wherein each path for cooling liquid comprises several sectionsconnected together by connecting ducts.
 4. Device according to claim 2,wherein each path for cooling liquid is approximately in the form of aspiral.
 5. Device according to claim 2, wherein said two exchangers arelocated one on each side of a rotor of said device.
 6. The deviceaccording to claim 1, wherein each path for circulating the coolingliquid comprises a plurality of concentric sections connected togetherby connecting ducts.
 7. Device according to claim 6, wherein each pathfor cooling liquid is approximately in the form of a spiral.
 8. Deviceaccording to claim 1, characterized in that each path (51) for coolingliquid is approximately in the form of a spiral.
 9. Device according toclaim 8, wherein each path for the cooling liquid is produced at leastpartially by winding a tube of approximately constant cross section. 10.Device according to claim 1, characterized in that each path for thecooling liquid is produced at least partially by machining.
 11. Deviceaccording to claim 10, wherein each path for the cooling liquid isproduced at least partially by winding a tube of approximately constantcross section.
 12. Device according to claim 1, characterized in thateach path (51) for the cooling liquid is produced at least partially bywinding a tube (51) of approximately constant cross section.
 13. Thedevice according to claim 1, wherein said two heat exchangers arelocated one on each side of the rotor of said device.
 14. Deviceaccording to claim 1, characterized in that said two exchangers areapproximately identical.
 15. The device according to claim 1, whereinsaid two heat exchangers are located symmetrically with respect to anaxis of symmetry which is perpendicular to the axis of rotation of therotor.
 16. The device according to claim 15, wherein respective coolingliquid inlets and outlets of said two heat exchangers are arrangedsymmetrically with respect to said axis of symmetry.
 17. The deviceaccording to claim 1, further comprising: a device body containing saidtwo heat exchangers; a plurality of oscillating bearings supporting saidbody; and a shaft mounted so as to rotate with respect to said body,said shaft carrying said rotor and being couplable to an engine fordriving said shaft, said body comprising a load sensor for measuring arighting torque exerted on said body so as to determine a torque of saidengine, said load sensor being secured to a stationary bed by ananti-rotation arm.
 18. An eddy-current brake device comprising: at leastone heat exchanger for dissipating thermal energy formed by eddycurrents during braking, wherein said at least one heat exchangercomprises: an exchanger plate; a plurality of concentric grooves in saidexchanger plate; a plurality of passages, one of said plural passagesconnecting a first and second groove of said plural concentric grooves;and a plurality of blocking pegs in each of said plural concentricgrooves to permit spiral fluid flow in essentially one direction. 19.The device as claimed in claim 18, wherein each of said plural blockingpegs are adjacent an inlet of each respective said plural passages. 20.The device as claimed in claim 18, wherein each said plural passagesform an oblique angle with said plural concentric grooves.